lbaas on a network without gateway

Not only check the gateway_ip in subnet, but also
check the host_routes. If there is a default route in host_routes
then the next hop of the default route is also a gateway_ip.

Closes-Bug: 1300017
DocImpact

Change-Id: Id3ae04bfa6cefb1c030857894f0594828b8e856d
Fix H302 violations

H302 violation is reported by flake8 when importing separated objects from
modules instead of importing the whole module.
e.g.   from package.module import function
       function()
is changed to
       from package import module
       module.function()

Change-Id: Ifbf31b52316d3cade40743752a49ce700f384a21
Closes-Bug: #1291032
Merge "Fix H302 violations"
Merge "lbaas on a network without gateway"